Apparatus and method for performing line analysis of continuous data signals

ABSTRACT

An apparatus for performing an optical line analysis of continuous data signals. The apparatus comprise a phase position processor for computing a phase early/late indicator; a phase control code processor for computing a difference phase indicator; a frequency extractor for computing a low frequency jitter indicator; and a statistical calculator for computing a plurality of statistical measures regarding frequency and amplitude components of a jitter of an input continuous data signal, wherein the statistical measures are computed based on one of the phase early/late information indicator, the difference phase indicator, or the low frequency jitter indicator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/049,771 filed on May 2, 2008, the contents of which are hereinincorporated by reference.

TECHNICAL FIELD

The present invention relates generally to an apparatus and a method formeasurement of the quality of signals transmitted in opticaltransmission systems.

BACKGROUND OF THE INVENTION

Many communication networks that provide high bit-rate transport over ashared medium are characterized by non-continuous, or burst mode, datatransmission in the upstream direction and continuous data transmissionin the downstream direction. An example of such a network is a passiveoptical network (PON) 100 schematically shown in FIG. 1. A typical PON100 includes a plurality of optical network units (ONUs) 120-1 through120-M coupled to an optical line terminal (OLT) 130 via a passiveoptical splitter 140. Since all ONUs function in like manner, they willbe collectively referred to by the reference numeral 120 in thefollowing description unless reference is made to a specific ONU.

Traffic data transmission is performed over two optical wavelengths, onefor the downstream direction and another for the upstream direction.Thus, downstream transmission from the OLT 130 is broadcast to all theONUs 120. Each ONU 120 filters its respective data according to, forexample, pre-assigned labels. Transmission from an ONU 120 to the OLT130 is in the form of a burst. The OLT 130 continuously transmitsdownstream data to the ONUs 120 and receives upstream burst data sent toOLT 130 from ONUs 120. The OLT 130 broadcasts data to the ONUs 120 alonga common channel so that all the ONUs 120 receive the same data. An ONU120 transmits data to the OLT 130 during different time slots allocatedby the OLT 130. That is, the OLT 130 is aware of the exact arrival timeof data and the identity of a transmitting ONU 120.

A PON is typically designed with varied lengths of optical links,splits, cost driven optics, and other physical consideration. Thus atypical PON suffers from optical aberrations influencing the signals.Therefore, appropriate signal processing is required in order to recoverthe original signal from the received signal and to avoid errors duringtransmission.

An optical signal sent from an OLT 130 is received by a receiver in theONU 120 and converted into an analog electrical signal. The ONU'sreceiver uses a clock and data recovery (CDR) circuit to generate aclock corresponding to the incoming data, thereby correctly retiming thedigital incoming data. After recovering the data, a forward errorcorrection mechanism may be utilized to detect and correct errors in thereceived data and to provide an assessment of the signal quality.However, during the recovery process, essential information, such as eyedistortion, frequency movement, phase information, jitter, and othereffects are discarded, and thus the quality of the input signal cannotbe correctly measured. Therefore, assessment of the signal quality isnecessary prior to recovering the signals.

In PON systems there is an increasing demand to perform an optical lineanalysis to determine the root cause of the PON failures or performancedegradation. Results of an optical line analysis can enable PONoperators to perform optical layer supervision. The optical supervisionallows more efficient operation and maintenance of PON networks, forexample, by not sending technicians if the PON system works properly,dispatching the correct technician if a problem is detected, orproviding correct diagnostics to the technician.

Optical line analysis of signals can be performed only on signals thatare not fully recovered and cannot be performed using conventionaltechniques for detecting the errors in the received signals.

Therefore, it would be advantageous to provide a solution for performingan optical line analysis in passive optical networks.

SUMMARY OF THE INVENTION

Certain embodiments of the invention include an apparatus for performingan optical line analysis of continuous data signals. The apparatuscomprises a phase position processor for computing a phase early/lateindicator; a phase control code processor for computing a differencephase indicator; a frequency extractor for computing a low frequencyjitter indicator; and a statistical calculator for computing a pluralityof statistical measures regarding frequency and amplitude components ofa jitter of an input continuous data signal, wherein the statisticalmeasures are computed based on one of the phase early/late informationindicator, the difference phase indicator, or the low frequency jitterindicator.

Certain embodiments of the invention also include a method forperforming an optical line analysis of continuous data signals. Themethod comprising computing a phase early/late indicator based on aphase position of an input continuous data signal relative to samplingclock signals; computing a difference phase indicator based on an inputphase control code; computing a low frequency jitter indicator based onan input phase control code; and computing a plurality of statisticalmeasures regarding frequency and amplitude components of a jitter of theinput continuous data signal, wherein the statistical measures arecomputed based on one of the phase early/late information indicator, thedifference phase indicator, or the low frequency jitter indicator; andanalyzing the plurality of statistical measures to detect variousfailure indicators indicating various malfunctions of the optical line.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter that is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other objects, features andadvantages of the invention will be apparent from the following detaileddescription taken in conjunction with the accompanying drawings.

FIG. 1 is a schematic diagram of a passive optical network;

FIG. 2 is a block diagram of a CDR circuit used to describe certainprinciples of the invention;

FIG. 3 is a block diagram of the SAB constructed in accordance with oneembodiment of the invention;

FIG. 4 is a block diagram of the frequency extractor constructed inaccordance with one embodiment of the invention;

FIG. 5 is a graph showing the jitter analysis in different frequencyranges; and

FIG. 6 is a flowchart describing a method for measuring the jitter of acontinuous signal.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments disclosed by the invention are only examples of the manypossible advantageous uses and implementations of the innovativeteachings presented herein. In general, statements made in thespecification of the present application do not necessarily limit any ofthe various claimed inventions. Moreover, some statements may apply tosome inventive features but not to others. In general, unless otherwiseindicated, singular elements may be in plural and vice versa with noloss of generality. In the drawings, like numerals refer to like partsthrough several views.

FIG. 2 shows a non-limiting block diagram of a CDR circuit 200 used todescribe certain principles of the invention. The CDR circuit 200 isbased on an over-sampling technique and includes an over-sampler 210, aphase interpolator 220, a phase estimation unit (PEU) 230, a digitalfilter 240, a statistical accumulation block (SAB) 250, and a processor260. The phase interpolator 220 is used to generate a number Q (where Qis an integer number greater than 1) of sampling clock signals 201 atthe oscillating frequency provided by a reference clock 202, generatedby, for example, an oscillator or a recovered clock phase locked loop(PLL) device (not shown in FIG. 2). When the recovered PLL is used oneof the signal 201 is input to a PLL (not shown). Each signal 201 isshifted in phase by a factor 1/Q of the clock cycle with respect to thepreceding signal. The over-sampler 210 receives an input continuous datasignal 203 and using the sampling clock signals 201, the over-sampler210 generates a bit stream 204 of the recovered data. The data signalmay be sent over a PON or a high-speed serial interface.

The PEU 230 receives the bit stream 204, computes and outputs a phaseposition of the input data signal 203 relatively to signals 201. Thatis, the PEU 230 provides early/late phase information related to phaseof the input data signal 203. The phase position is input into the SAB250 and the digital filter 240 which computes a phase control code. Thephase control code is used to set the phase interpolator 220 with thephase error control code to generate a correct sampling clock signal forfuture sampling of the input data signal 201. In accordance with anembodiment of the invention setting the phase interpolator 220 with aphase information may be achieved using a phase retrieval unit or aphase mover, as disclosed in co-pending U.S. patent application Ser. No.11/604,748 entitled “Burst mode clock and data recovery circuit andmethod” assigned to common assignee and is hereby incorporated byreference for all that it contains.

The phase control code is also fed to the SAB 250. The SAB 250 performsjitter analysis using the input phase position and phase control codeinformation and outputs statistical measures of the jitter frequency andamplitude components in different frequency ranges starting from directcurrent (DC) to an order of the frequency of the input data. The SAB 250also computes statistical measures related to the frequency deviation ofthe reference clock signal 202 and a clock of the input data signal 203.The statistical measures may include, but are not limited to, a maximumvalue, a minimum value, an average value, an absolute value, a standarddeviation value and so on. The operation of the SAB 250 is described indetail below.

The processor 260 aggregates the statistical measures and analyzes thegathered information to detect phenomena that may indicate potentialfailures in the PON (or any other transport means or transmission line).Failures indicators are reported to the PON operator that may determine,based on indicators reported by other ONUs, the root cause of failures.The processor 260 may be a processor of the ONU. The gatheredstatistical measures and the processing results may be saved in a memory(not shown).

As a non-limiting example for a failure indicator is a reference clockwhich is not within a specified locking range of the input data signal.This may indicate on a potential malfunction of the ONU, probably due todegradation of a local oscillator circuit (for the reference clock basedPLL), a local narrowband PLL (NB-PLL) that has erratic behavior or aproblem in the OLT (for the recovered clock PLL reference clock). Thisindication may also be reported to a PON operator which can determine,based on reports of other ONUs, if the same problem occurs on all ONUs,certain ONUs, or a single ONU. Using this information the operator candetect the root cause of the unsynchronized clocks problem. As anotherexample, low frequency jitter (e.g., signal having a frequency below 2KHz) measured by an ONU can indicate either noisy transmission by theOLT (if such phenomenon is reported by all ONUs) or an unstable powersource of the ONU. Yet another non-limiting example for a failureindicator, the SAB 250 together with the processor 260 can detect jitterin the acoustic band (e.g., frequency band between 1 KHz and 50 KHz).This may indicate acoustic interferences on the optical line typicallycaused by winds, noise or pressure that is being put on the fiber (e.g.,construction work in the vicinity of the fiber, trains crossing thefiber, and so on). Other interferences may be detected by analyzing highjitter frequency components (e.g., 1 MHz and above).

It should be apparent to one of ordinarily skill in the art that thetype of failures described herein are merely examples and other types offailures may be detected using the statistical information generated bythe SAB 250. Furthermore, the statistical information may be furtherprocessed together with other PON indicators collected, for example, bythe OLT. An example for an optical line analysis performed by an OLT isdescribed in co-pending U.S. patent application Ser. No. 11/961,789entitled “Apparatus and method for measuring the quality of burstsignals and performing optical line diagnostics” assigned to commonassignee and is hereby incorporated by reference for all that itcontains.

FIG. 3 shows a non-limiting and exemplary block diagram of the SAB 250constructed in accordance with one embodiment of the invention. The SAB250 includes a phase-position processor 310, a phase control-codeprocessor 320, a frequency extractor 330, a multiplexer 340, a downsampler 350, a filter 360, and a statistical calculator 370. The phaseposition 310 processes the information provided by the PEU 230 andoutputs the total number of “early” or “late” bits with respect to phasecomponents of the input signal. For example, if in an input bit stream 6bits were “late” in phase and 2 bits “early” in phase the total is 4early bits.

The processor 320 receives the current phase control code from thedigital filter 240 and the expected phase code or the previous phasecode from the frequency extractor 330 and outputs a normal binaryrepresentation calculated difference (Out_Diff) codes. An Out-Diff codemay be the difference between the current phase control code and theprevious code or the difference between the expected code and theprevious code. The phase control code is a data word coded using, forexample, a binary code. The Out_Diff is the normal binary representationof the difference. A binary equivalent of the phase control code isoutput to the frequency extractor 330, which extracts semi-constantphase shift of a very low jitter frequency. Typically, the low jitterfrequency is not above 2 KHz. The frequency extractor 330 also measuresthe frequency deviation (difference) between a clock of the input signal203 and the reference clock 202.

A non-limiting block diagram of the frequency extractor 330 is providedin FIG. 4. The frequency extractor 330 is a closed loop circuit thatincludes two configurable amplifiers 410 and 420 that together enable tomeasure both frequency shifts and low frequency drifts in inputssignals. The statistical block 440 computes at least one of minimum,maximum, absolute and average values of the frequency deviation betweenthe reference clock 202 and a clock the input signal 203 shown in FIG.2. The output of the statistical block 440 is fed to the processor 260.

Referring back to FIG. 3, the multiplexer 340 selects which phaseindicators, i.e., the outputs of the phase position processor 310, phasecontrol processor 320 and frequency extractor 330, to be input to thedown sampler 350 for further processing. As shown in FIG. 5 each ofthese phase indicators allows analyzing the jitter of input signals in adifferent frequency range. As a non-limiting example the frequencyextractor 330 output can be used to analyze jitter in a frequency rangeof up to 2 KHz, the Out_Diff value can be used to measure jitter in afrequency band between 2 KHz and 5 MHz and the output of the totalearly/late number is used for analyzing jitter between 3 MHz to maximumworking frequency of the filter 360.

The down sampler 350 is a configurable unit that adapts the frequency ofthe selected output to a frequency work point of the filter 360. Thisperforms by averaging the data signal according to a configurableparameter. The filter 360 is a configurable filter that passes onlysignal in a predefined frequency window to be analyzed. In a preferredembodiment the filter 360 maybe a configurable filter structure that canimplement either an Infinite impulse response (IIR) a finite impulseresponse (FIR) and to perform single bin calculation for specificfrequency (amplitude) detection.

The statistical calculator 370 computes, in real time, statisticalmeasures related to the frequency and amplitude components of the jitterwithin a frequency window set by the filter 360. As mentioned above thestatistical measures include, but are not limited to, minimum, maximum,absolute, and average values of the frequency component of the jitter aswell as minimum, maximum, absolute, and average values of the frequencycomponent of the jitter. In accordance with an embodiment of theinvention the SAB 250 can be utilized to measure a transferred jitter ofa signal transmitter by an ONU based on at least the phase control code.

FIG. 6 shows a non-limiting and exemplary flowchart 600 describing themethod for measuring the jitter of continuous data signals implementedin accordance with an embodiment of the invention. At S610 an inputcontinuous signal is over-sampled to generate a sequence of bit streams.At S620, phase information is generated based on the over-sampledstream. Specifically, the phase information includes a phase position(or early/late information) produced by the PEU 230 and a phase controlcode generated by the digital filter 240. At S630, a low frequencyjitter is extracted from the phase control code. At S640, one of thephase indicators including the phase position, phase control code, lowfrequency jitter are selected to be analyzed. Each of these indicatorsis used to measure the jitter in a different frequency range. The phaseposition information can use to analyze jitter in a frequency of above 2MHz, the phase control code information can be utilized to measurejitter in a frequency range of 1 MHz to 5 MHz, the low frequency jittercan be measured in frequencies between DC to 2 KHz. At S650 variousstatistical measures including, for example, average, absolute, minimum,and maximum values of the frequency and amplitude jitter components ofthe input. At S660, the statistical measures are gathered and preferablysaved in memory to be further processed to detect failures indicators.

The invention described herein references an exemplary embodiment wherea line analysis is performed in optical networks. However, it would beapparent to one of ordinary skill in art that the line analysis is basedon a jitter analysis of continuous data signals. Therefore, one ofordinary skill in the art can adapt the disclosed invention to performline analysis on continuous data received on, for example, a high-speedserial bus including, but not limited to, a Serial ATA (SATA) bus, aPeripheral Component Interconnect express (PCIe) bus, a Universal SerialBus (USB), a Hypertransport bus, and an Infiniband bus, and the like.

The principles of the invention may be implemented as hardware,firmware, software or any combination thereof. Moreover, the software ispreferably implemented as an application program tangibly embodied on aprogram storage unit or computer readable medium. The applicationprogram may be uploaded to, and executed by, a machine comprising anysuitable architecture. Preferably, the machine is implemented on acomputer platform having hardware such as one or more central processingunits (“CPUs”), a memory, and input/output interfaces. The computerplatform may also include an operating system and microinstruction code.The various processes and functions described herein may be either partof the microinstruction code or part of the application program, or anycombination thereof, which may be executed by a CPU, whether or not suchcomputer or processor is explicitly shown. In addition, various otherperipheral units may be connected to the computer platform such as anadditional data storage unit and a printing unit.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the principlesof the invention and the concepts contributed by the inventor tofurthering the art, and are to be construed as being without limitationto such specifically recited examples and conditions. Moreover, allstatements herein reciting principles, aspects, and embodiments of theinvention, as well as specific examples thereof, are intended toencompass both structural and functional equivalents thereof.Additionally, it is intended that such equivalents include bothcurrently known equivalents as well as equivalents developed in thefuture, i.e., any elements developed that perform the same function,regardless of structure.

1. An apparatus for performing an optical line analysis of continuousdata signals, comprising: a phase position processor for computing aphase early/late indicator; a phase control code processor for computinga difference phase indicator; a frequency extractor for computing a lowfrequency jitter indicator; a statistical calculator for computing aplurality of statistical measures regarding frequency and amplitudecomponents of a jitter of an input continuous data signal, wherein thestatistical measures are computed based on one of the phase early/lateinformation indicator, the difference phase indicator, and the lowfrequency jitter indicator; and a processor for aggregating theplurality of statistical measures and analyzing the aggregated data todetect various failure indicators indicating various malfunctions of theoptical line, wherein the optical line is an optical fiber of a passiveoptical network (PON), wherein the optical fiber connects an opticalline terminal (OLT) to a plurality of optical network units (ONUs)through an optical splitter.
 2. The apparatus of claim 1, furthercomprising: a down sampler for down sampling one of the phase early/lateinformation indicator, the difference phase indicator, and the lowfrequency jitter indicator to a frequency work point of a filter; and afilter for filtering the down sampled signal to a predefined frequencywindow in which the input continuous data signal is to be analyzed,wherein the output of the filter is fed to the statistical calculator.3. The apparatus of claim 1, wherein the phase position processorcomputes the phase early/late indicator based on a phase position of theinput continuous data signal relatively to sampling clock signals. 4.The apparatus of claim 1, wherein the phase control code processorcomputes the difference phase indicator based on an input phase controlcode.
 5. The apparatus of claim 1, wherein the frequency extractorcomputes the low frequency jitter indicator based on an input phasecontrol code, wherein the low frequency jitter indicator is a binaryrepresentation of a frequency deviation between a reference clock and aclock of the input continuous data signal.
 6. The apparatus of claim 1,wherein the plurality of statistical measures include at least a maximumvalue, a minimum value, an absolute value, an average value, and astandard deviation value.
 7. The apparatus of claim 1, wherein thevarious failure indicators include at least: a reference clock that isnot within a specified locking range of the input continuous data signalindicating on a potential malfunction of an ONU or an OLT if the sameindication was detected for all ONUs; a low frequency measured jitter ofan input continuous data signal indicating on either noisy transmissionby the OLT or on an unstable power source of the ONU; and a measuredjitter of an input continuous data signal is within a acoustic bandindicating on acoustic interferences on the optical line.
 8. Theapparatus of claim 1, wherein the various failure indicators arereported to a PON operator.
 9. The apparatus of claim 1, wherein theoptical line carries data of a high-speed serial bus, wherein thehigh-speed serial bus comprises at least a Serial ATA (SATA) bus, aPeripheral Component Interconnect express (PCIe) bus, a Universal SerialBus (USB), a Hypertransport bus, and an Infiniband bus.
 10. Theapparatus of claim 1, is further capable to measure a transferred jitterof a signal transmitter by an ONU based on at least the phase controlcode.
 11. A method for performing an optical line analysis of continuousdata signals comprises: computing a phase early/late indicator based ona phase position of an input continuous data signal relative to samplingclock signals; computing a difference phase indicator based on an inputphase control code; computing a low frequency jitter indicator based onan input phase control code; and computing a plurality of statisticalmeasures regarding frequency and amplitude components of a jitter of theinput continuous data signal, wherein the statistical measures arecomputed based on one of the phase early/late information indicator, thedifference phase indicator, and the low frequency jitter indicator;analyzing the plurality of statistical measures to detect variousfailure indicators indicating various malfunctions of the optical line,wherein the optical line is an optical fiber of a passive opticalnetwork (PON), wherein the optical fiber connects an optical lineterminal (OLT) to a plurality of optical network units (ONUs) through anoptical splitter; down sampling one of the phase early/late informationindicator, the difference phase indicator, or the low frequency jitterindicator to a frequency work point of a filter; and filtering the downsampled signal to a predefined frequency window in which the inputcontinuous data signal is to be analyzed.
 12. The method of claim 11,wherein the various failure indicators include at least: a referenceclock that is not within a specified locking range of the inputcontinuous data signal indicating on a potential malfunction of an ONUor an OLT if the same indication was detected for all ONUs; a lowfrequency measured jitter of an input continuous data signal indicatingon either noisy transmission by the OLT or on an unstable power sourceof the ONU; and a measured jitter of an input continuous data signal iswithin a acoustic band indicating on acoustic interferences on theoptical line.
 13. The method of claim 11, wherein the various failureindicators are reported to a PON operator.
 14. A non-transitory computerreadable medium having stored thereon computer exactable code whenexecuted by a processor causing the processor to perform the process of:performing an optical line analysis of continuous data signalscomprises: computing a phase early/late indicator based on a phaseposition of an input continuous data signal relatively to sampling clocksignals; computing a difference phase indicator based on an input phasecontrol code; computing a low frequency jitter indicator based on aninput phase control code; and computing a plurality of statisticalmeasures regarding frequency and amplitude components of a jitter of theinput continuous data signal, wherein the statistical measures arecomputed based on one of the phase early/late information indicator, thedifference phase indicator, and the low frequency jitter indicator;analyzing the plurality of statistical measures to detect variousfailure indicators indicating on various malfunctions of the opticalline, wherein the optical line is an optical fiber of a passive opticalnetwork (PON), wherein the optical fiber connects an optical lineterminal (OLT) to a plurality of optical network units (ONUs) through anoptical splitter; down sampling one of the phase early/late informationindicator, the difference phase indicator, or the low frequency jitterindicator to a frequency work point of a filter; and filtering the downsampled signal to a predefined frequency window in which the inputcontinuous data signal is to be analyzed.